Security & Monitoring

Transcription

1 6 Security & Monitoring In a traditional wired network, access control is very straightforward: If a person has physical access to a computer or network hub, they can use (or abuse) the network resources. While software mechanisms are an important component of network security, limiting physical access to the network devices is the ultimate access control mechanism. Simply put, if all terminals and network components are only accessible to trusted individuals, the network can likely be trusted. The rules change significantly with wireless networks. While the apparent range of your access point may seem to be just a few hundred meters, a user with a high gain antenna may be able to make use of the network from several blocks away. Should an unauthorized user be detected, is impossible to simply trace the cable back to the user s location. Without transmitting a single packet, a nefarious user can even log all network data to disk. This data can later be used to launch a more sophisticated attack against the network. Never assume that radio waves simply stop at the edge of your property line. It is usually unreasonable to completely trust all users of the network, even on wired networks. Disgruntled employees, uneducated network users, and simple mistakes on the part of honest users can cause significant harm to network operations. As the network architect, your goal is to facilitate private communication between legitimate users of the network. While a certain amount of access control and authentication is necessary in any network, you have failed in your job if legitimate users find it difficult to use the network to communicate. There s an old saying that the only way to completely secure a computer is to unplug it, lock it in a safe, destroy the key, and bury the whole thing in con- 157

2 158 Chapter 6: Security & Monitoring crete. While such a system might be completely secure, it is useless for communication. When you make security decisions for your network, remember that above all else, the network exists so that its users can communicate with each other. Security considerations are important, but should not get in the way of the network s users. Physical security When installing a network, you are building an infrastructure that people depend on. Security measures exist to ensure that the network is reliable. For many installations, outages often occur due to human tampering, whether accidental or not. Networks have physical components, such as wires and boxes, which are easily disturbed. In many installations, people will not understand the purpose of the installed equipment, or curiosity may lead them to experiment. They may not realize the importance of a cable connected to a port. Someone may unplug an Ethernet cable so that they can connect their laptop for 5 minutes, or move a switch because it is in their way. A plug might be removed from a power bar because someone needs that receptacle. Assuring the physical security of an installation is paramount. Signs and labels will only be useful to those who can read your language. Putting things out of the way and limiting access is the best means to assure that accidents and tinkering do not occur. In less developed economies, proper fasteners, ties, or boxes will not be as easy to find. You should be able to find electrical supplies that will work just as well. Custom enclosures are also easy to manufacture and should be considered essential to any installation. It is often economical to pay a mason to make holes and install conduit. Where this would be an expensive option in the developed world, this type of labour intensive activity can be affordable in Southern countries. PVC can be embedded in cement walls for passing cable from room to room. This avoids the need to smash new holes every time a cable needs to be passed. Plastic bags can be stuffed into the conduit around the cables for insulation. Small equipment should be mounted on the wall and larger equipment should be put in a closet or in a cabinet. Switches Switches, hubs or interior access points can be screwed directly onto a wall with a wall plug. It is best to put this equipment as high as possible to reduce the chance that someone will touch the device or its cables.

3 Chapter 6: Security & Monitoring 159 Cables At the very least, cables should be hidden and fastened. It is possible to find plastic cable conduit that can be used in buildings. If you cannot find it, simple cable attachments can be nailed into the wall to secure the cable. This will make sure that the cable doesn't hang where it can be snagged, pinched or cut. It is preferable to bury cables, rather than to leave them hanging across a yard. Hanging wires might be used for drying clothes, or be snagged by a ladder, etc. To avoid vermin and insects, use plastic electrical conduit. The marginal expense will be well worth the trouble. The conduit should be buried about 30 cm deep, or below the frost level in cold climates. It is worth the extra investment of buying larger conduit than is presently required, so that future cables can be run through the same tubing. Consider labeling buried cable with a "call before you dig" sign to avoid future accidental outages. Power It is best to have power bars locked in a cabinet. If that is not possible, mount the power bar under a desk, or on the wall and use duct tape (or gaffer tape, a strong adhesive tape) to secure the plug into the receptacle. On the UPS and power bar, do not leave any empty receptacles. Tape them if necessary. People will have the tendency to use the easiest receptacle, so make these critical ones difficult to use. If you do not, you might find a fan or light plugged into your UPS; though it is nice to have light, it is nicer to keep your server running! Water Protect your equipment from water and moisture. In all cases make sure that your equipment, including your UPS is at least 30 cm from the ground, to avoid damage from flooding. Also try to have a roof over your equipment, so that water and moisture will not fall onto it. In moist climates, it is important that the equipment has proper ventilation to assure that moisture can be exhausted. Small closets need to have ventilation, or moisture and heat can degrade or destroy your gear. Masts Equipment installed on a mast is often safe from thieves. Nevertheless, to deter thieves and to keep your equipment safe from winds it is good to overengineer mounts. Painting equipment a dull white or grey color reflects the sun and makes it look plain and uninteresting. Panel antennas are often preferred because they are much more subtle and less interesting than dishes. Any installation on walls should be high enough to require a ladder to reach. Try choosing well-lit but not prominent places to put equipment. Also avoid anten-

4 160 Chapter 6: Security & Monitoring nae that resemble television antennae, as those are items that will attract interest by thieves, where a wifi antenna will be useless to the average thief. Threats to the network One critical difference between Ethernet and wireless is that wireless networks are built on a shared medium. They more closely resemble the old network hubs than modern switches, in that every computer connected to the network can see the traffic of every other user. To monitor all network traffic on an access point, one can simply tune to the channel being used, put the network card into monitor mode, and log every frame. This data might be directly valuable to an eavesdropper (including data such as , voice data, or online chat logs). It may also provide passwords and other sensitive data, making it possible to compromise the network even further. As we ll see later in this chapter, this problem can be mitigated by the use of encryption. Another serious problem with wireless networks is that its users are relatively anonymous. While it is true that every wireless device includes a unique MAC address that is supplied by the manufacturer, these addresses can often be changed with software. Even when the MAC address is known, it can be very difficult to judge where a wireless user is physically located. Multipath effects, high-gain antennas, and widely varying radio transmitter characteristics can make it impossible to determine if a malicious wireless user is sitting in the next room or is in an apartment building a mile away. While unlicensed spectrum provides a huge cost savings to the user, it has the unfortunate side effect that denial of service (DoS) attacks are trivially simple. By simply turning on a high powered access point, cordless phone, video transmitter, or other 2.4 GHz device, a malicious person could cause significant problems on the network. Many network devices are vulnerable to other forms of denial of service attacks as well, such as disassociation flooding and ARP table overflows. Here are several categories of individuals who may cause problems on a wireless network: Unintentional users. As more wireless networks are installed in densely populated areas, it is common for laptop users to accidentally associate to the wrong network. Most wireless clients will simply choose any available wireless network when their preferred network is unavailable. The user may then make use of this network as usual, completely unaware that they may be transmitting sensitive data on someone else s network. Malicious people may even take advantage of this by setting up access points in strategic locations, to try to attract unwitting users and capture their data.

5 Chapter 6: Security & Monitoring 161 The first step in avoiding this problem is educating your users, and stressing the importance of connecting only to known and trusted networks. Many wireless clients can be configured to only connect to trusted networks, or to ask permission before joining a new network. As we will see later in this chapter, users can safely connect to open public networks by using strong encryption. War drivers. The war driving phenomenon draws its name from the popular 1983 hacker film, War Games. War drivers are interested in finding the physical location of wireless networks. They typically drive around with a laptop, GPS, and omnidirectional antenna, logging the name and location of any networks they find. These logs are then combined with logs from other war drivers, and are turned into graphical maps depicting the wireless footprint of a particular city. The vast majority of war drivers likely pose no direct threat to networks, but the data they collect might be of interest to a network cracker. For example, it might be obvious that an unprotected access point detected by a war driver is located inside a sensitive building, such as a government or corporate office. A malicious person could use this information to illegally access the network there. Arguably, such an AP should never have been set up in the first place, but war driving makes the problem all the more urgent. As we will see later in this chapter, war drivers who use the popular program NetStumbler can be detected with programs such as Kismet. For more information about war driving, see sites such as or Rogue access points. There are two general classes of rogue access points: those incorrectly installed by legitimate users, and those installed by malicious people who intend to collect data or do harm to the network. In the simplest case, a legitimate network user may want better wireless coverage in their office, or they might find security restrictions on the corporate wireless network too difficult to comply with. By installing an inexpensive consumer access point without permission, the user opens the entire network up to potential attacks from the inside. While it is possible to scan for unauthorized access points on your wired network, setting a clear policy that prohibits them is very important. The second class of rogue access point can be very difficult to deal with. By installing a high powered AP that uses the same ESSID as an existing network, a malicious person can trick people into using their equipment, and log or even manipulate all data that passes through it. Again, if your users are trained to use strong encryption, this problem is significantly reduced. Eavesdroppers. As mentioned earlier, eavesdropping is a very difficult problem to deal with on wireless networks. By using a passive monitoring tool (such as Kismet), an eavesdropper can log all network data from a great distance away, without ever making their presence known. Poorly

6 162 Chapter 6: Security & Monitoring encrypted data can simply be logged and cracked later, while unencrypted data can be easily read in real time. If you have difficulty convincing others of this problem, you might want to demonstrate tools such as Etherpeg ( or Driftnet ( These tools watch a wireless network for graphical data, such as GIF and JPEG files. While other users are browsing the Internet, these tools simply display all graphics found in a graphical collage. I often use tools such as this as a demonstration when lecturing on wireless security. While you can tell a user that their is vulnerable without encryption, nothing drives the message home like showing them the pictures they are looking at in their web browser. Again, while it cannot be completely prevented, proper application of strong encryption will discourage eavesdropping. This introduction is intended to give you an idea of the problems you are up against when designing a wireless network. Later in this chapter, we will look at tools and techniques that will help you to mitigate these problems. Authentication Before being granted access to network resources, users should first be authenticated. In an ideal world, every wireless user would have an identifier that is unique, unchangeable, and cannot be impersonated by other users. This turns out to be a very difficult problem to solve in the real world. The closest feature we have to a unique identifier is the MAC address. This is the 48-bit number assigned by the manufacturer to every wireless and Ethernet device. By employing mac filtering on our access points, we can authenticate users based on their MAC address. With this feature, the access point keeps an internal table of approved MAC addresses. When a wireless user tries to associate to the access point, the MAC address of the client must be on the approved list, or the association will be denied. Alternately, the AP may keep a table of known bad MAC addresses, and permit all devices that are not on the list. Unfortunately, this is not an ideal security mechanism. Maintaining MAC tables on every device can be cumbersome, requiring all client devices to have their MAC addresses recorded and uploaded to the APs. Even worse, MAC addresses can often be changed in software. By observing MAC addresses in use on a wireless network, a determined attacker can spoof (impersonate) an approved MAC address and successfully associate to the AP. While MAC filtering will prevent unintentional users and even most curious individuals from accessing the network, MAC filtering alone cannot prevent attacks from determined attackers.

7 Chapter 6: Security & Monitoring 163 MAC filters are useful for temporarily limiting access from misbehaving clients. For example, if a laptop has a virus that sends large amounts of spam or other traffic, its MAC address can be added to the filter table to stop the traffic immediately. This will buy you time to track down the user and fix the problem. Another popular authentication feature of wireless the so-called closed network. In a typical network, APs will broadcast their ESSID many times per second, allowing wireless clients (as well as tools such as NetStumbler) to find the network and display its presence to the user. In a closed network, the AP does not beacon the ESSID, and users must know the full name of the network before the AP will allow association. This prevents casual users from discovering the network and selecting it in their wireless client. There are a number of drawbacks to this feature. Forcing users to type in the full ESSID before connecting to the network is error prone and often leads to support calls and complaints. Since the network isn t obviously present in site survey tools like NetStumbler, this can prevent your networks from showing up on war driving maps. But it also means that other network builders cannot easily find your network either, and specifically won t know that you are already using a given channel. A conscientious neighbor may perform a site survey, see no nearby networks, and install their own network on the same channel you are using. This will cause interference problems for both you and your neighbor. Finally, using closed networks ultimately adds little to your overall networks security. By using passive monitoring tools (such as Kismet), a skilled user can detect frames sent from your legitimate clients to the AP. These frames necessarily contain the network name. A malicious user can then use this name to associate to the access point, just like a normal user would. Encryption is probably the best tool we have for authenticating wireless users. Through strong encryption, we can uniquely identify a user in a manner that is very difficult to spoof, and use that identity to determine further network access. Encryption also has the benefit of adding a layer of privacy by preventing eavesdroppers from easily watching network traffic. The most widely employed encryption method on wireless networks is WEP encryption. WEP stands for wired equivalent privacy, and is supported by virtually all a/b/g equipment. WEP uses a shared 40-bit key to encrypt data between the access point and client. The key must be entered on the APs as well as on each of the clients. With WEP enabled, wireless clients cannot associate with the AP until they use the correct key. An eavesdropper listening to a WEP-enabled network will still see traffic and MAC addresses, but the data payload of each packet is encrypted. This provides a fairly good authentication mechanism while also adding a bit of privacy to the network.

8 164 Chapter 6: Security & Monitoring WEP is definitely not the strongest encryption solution available. For one thing, the WEP key is shared between all users. If the key is compromised (say, if one user tells a friend what the password is, or if an employee is let go) then changing the password can be prohibitively difficult, since all APs and client devices need to be changed. This also means that legitimate users of the network can still eavesdrop on each others traffic, since they all know the shared key. The key itself is often poorly chosen, making offline cracking attempts feasible. Even worse, the implementation of WEP itself is broken in many access points, making it even easier to crack some networks. While manufacturers have implemented a number of extensions to WEP (such as longer keys and fast rotation schemes), these extensions are not part of the standard, and generally will not interoperate between equipment from different manufacturers. By upgrading to the most recent firmware for all of your wireless devices, you can prevent some of the early attacks found in WEP. WEP can still be a useful authentication tool. Assuming your users can be trusted not to give away the password, you can be fairly sure that your wireless clients are legitimate. While WEP cracking is possible, it is beyond the skill of most users. WEP is quite useful for securing long distance point-topoint links, even on generally open networks. By using WEP on such a link, you will discourage others from associating to the link, and they will likely use other available APs instead. Think of WEP as a handy keep out sign for your network. Anyone who detects the network will see that a key is required, making it clear that they are not welcome to use it. WEPs greatest strength is its interoperability. In order to comply with the standards, all wireless devices support basic WEP. While it isn t the strongest method available, it is certainly the most commonly implemented encryption feature. We will look at other more advanced encryption techniques later in this chapter. For more details about the state of WEP encryption, see these papers: Another data-link layer authentication protocol is Wi-Fi Protected Access, or WPA. WPA was created specifically to deal with the known problems with WEP mentioned earlier. It provides a significantly stronger encryption scheme, and can use a shared private key, unique keys assigned to each user, or even SSL certificates to authenticate both the client and the access point. Authentication credentials are checked using the 802.1X protocol,

9 Chapter 6: Security & Monitoring 165 which can consult a third party database such as RADIUS. Through the use of Temporal Key Integrity Protocol (TKIP), keys can be rotated quickly over time, further reducing the likelihood that a particular session can be cracked. Overall, WPA provides significantly better authentication and privacy than standard WEP. WPA requires fairly recent access point hardware and up-to-date firmware on all wireless clients, as well as a substantial amount of configuration. If you are installing a network in a setting where you control the entire hardware platform, WPA can be ideal. By authenticating both clients and APs, it solves the rogue access point problem and provides many significant advantages over WEP. But in most network settings where the vintage of hardware is mixed and the knowledge of wireless users is limited, WPA can be a nightmare to install. It is for this reason that most sites continue to use WEP, if encryption is used at all. Captive portals One common authentication tool used on wireless networks is the captive portal. A captive portal uses a standard web browser to give a wireless user the opportunity to present login credentials. It can also be used to present information (such as an Acceptable Use Policy) to the user before granting further access. By using a web browser instead of a custom program for authentication, captive portals work with virtually all laptops and operating systems. Captive portals are typically used on open networks with no other authentication methods (such as WEP or MAC filters). To begin, a wireless user opens their laptop and selects the network. Their computer requests a DHCP lease, which is granted. They then use their web browser to go to any site on the Internet. Internet Captive portal Login: Figure 6.1: The user requests a web page and is redirected. Instead of receiving the requested page, the user is presented with a login screen. This page can require the user to enter a user name and password, simply click a login button, type in numbers from a pre-paid ticket, or enter any other credentials that the network administrators require. The user then

10 166 Chapter 6: Security & Monitoring enters their credentials, which are checked by the access point or another server on the network. All other network access is blocked until these credentials are verified. HTTP request waiting Internet User: joe Password: secret Captive portal Authentication service Figure 6.2: The user s credentials are verified before further network access is granted. The authentication server can be the access point itself, another machine on the local network, or a server anywhere on the Internet. Once authenticated, the user is permitted to access network resources, and is typically redirected to the site they originally requested. Redirect to Internet Credentials verified. Captive portal Authentication service Figure 6.3: After authenticating, the user is permitted to access the rest of the network. Captive portals provide no encryption for the wireless users, instead relying on the MAC and IP address of the client as a unique identifier. Since this is not necessarily very secure, many implementations will require the user to re-authenticate periodically. This can often be automatically done by minimizing a special pop-up browser window when the user first logs in. Since they do not provide strong encryption, captive portals are not a very good choice for networks that need to be locked down to only allow access

11 Chapter 6: Security & Monitoring 167 from trusted users. They are much more suited to cafes, hotels, and other public access locations where casual network users are expected. In public or semi-public network settings, encryption techniques such as WEP and WPA are effectively useless. There is simply no way to distribute public or shared keys to members of the general public without compromising the security of those keys. In these settings, a simple application such as a captive portal provides a level of service somewhere between completely open and completely closed. Popular hotspot projects Chillispot ( Chillispot is a captive portal designed to authenticate against an existing user credentials database, such as RADUIS. Combined with the application phpmyprepaid, pre-paid ticket based authentication can be implemented very easily You can download phpmyprepaid from WiFi Dog ( WiFi Dog provides a very complete captive portal authentication package in very little space (typically under 30kb). From a user s perspective, it requires no pop-up or javascript support, allowing it to work on a wider variety of wireless devices. m0n0wall ( m0n0wall is a complete embedded operating system based on FreeBSD. It includes a captive portal with RADIUS support, as well as a PHP web server. NoCatSplash ( provides a customizable splash page to your users, requiring them to click a login button before using the network. This is useful for identifying the operators of the network and displaying rules for network access. It provides a very easy solution in situations where you need to provide users of an open network with information and an acceptable use policy. Privacy Most users are blissfully unaware that their private , chat conversations, and even passwords are often sent in the clear over dozens of untrusted networks before arriving at their ultimate destination on the Internet. However mistaken they may be, users still typically have some expectation of privacy when using computer networks. Privacy can be achieved, even on untrusted networks such as public access points and the Internet. The only proven effective method for protecting privacy is the use of strong end-to-end encryption.

12 168 Chapter 6: Security & Monitoring Encryption techniques such as WEP and WPA attempt to address the privacy issue at layer two, the data-link layer. This does protect against eavesdroppers listening in on the wireless connection, but this protection ends at the access point. If the wireless client uses insecure protocols (such as POP or simple SMTP for receiving and sending ), then users beyond the AP can still log the session and see the sensitive data. As mentioned earlier, WEP also suffers from the fact that it uses a shared private key. This means that legitimate wireless users can eavesdrop on each other, since they all know the private key. By using encryption to the remote end of the connection, users can neatly sidestep the entire problem. These techniques work well even on untrusted public networks, where eavesdroppers are listening and possibly even manipulating data coming from the access point. To ensure data privacy, good end-to-end encryption should provide the following features: Verified authentication of the remote end. The user should be able to know without a doubt that the remote end is who it claims to be. Without authentication, a user could give sensitive data to anyone claiming to be the legitimate service. Strong encryption methods. The encryption algorithm should stand up to public scrutiny, and not be easily decrypted by a third party. There is no security in obscurity, and strong encryption is even stronger when the algorithm is widely known and subject to peer review. A good algorithm with a suitably large and protected key can provide encryption that is unlikely to be broken by any effort in our lifetimes using current technology. Public key cryptography. While not an absolute requirement for end-toend encryption, the use of public key cryptography instead of a shared key can ensure that an individual's data remains private, even if the key of another user of the service is compromised. It also solves certain problems with distributing keys to users over untrusted networks. Data encapsulation. A good end-to-end encryption mechanism protects as much data as possible. This can range from encrypting a single transaction to encapsulation of all IP traffic, including DNS lookups and other supporting protocols. Some encryption tools simply provide a secure channel that other applications can use. This allows users to run any program they like and still have the protection of strong encryption, even if the programs themselves don t support it. Be aware that laws regarding the use of encryption vary widely from place to place. Some countries treat encryption as munitions, and may require a permit, escrow of private keys, or even prohibit its use altogether. Before

13 Chapter 6: Security & Monitoring 169 implementing any solution that involves encryption, be sure to verify that use of this technology is permitted in your local area. In the following sections, we ll take a look at some specific tools that can provide good protection for your users data. SSL The most widely available end-to-end encryption technology is Secure Sockets Layer, known simply as SSL. Built into virtually all web browsers, SSL uses public key cryptography and a trusted public key infrastructure (PKI) to secure data communications on the web. Whenever you visit a web URL that starts with https, you are using SSL. The SSL implementation built into web browsers includes a collection of certificates from trusted sources, called certificate authorities (CA). These certificates are cryptographic keys that are used to verify the authenticity of websites. When you browse to a website that uses SSL, the browser and the server first exchange certificates. The browser then verifies that the certificate provided by the server matches its DNS host name, that it has not expired, and that it is signed by a trusted certificate authority. The server optionally verifies the identity of the browser s certificate. If the certificates are approved, the browser and server then negotiate a master session key using the previously exchanged certificates to protect it. That key is then used to encrypt all communications until the browser disconnects. This kind of data encapsulation is known as a tunnel. The tunnel can terminate anywhere on the Internet. Eavesdroppers can watch unencrypted wireless traffic. Internet Access point Wireless traffic is protected by an encrypted tunnel. Figure 6.4: Eavesdroppers must break strong encryption to monitor traffic over an encrypted tunnel. The conversation inside the tunnel is identical to any other unencrypted conversation. The use of certificates with a PKI not only protects the communication from eavesdroppers, but also prevents so-called man-in-the-middle (MITM) at-

14 170 Chapter 6: Security & Monitoring tacks. In a man-in-the-middle attack, a malicious user intercepts all communication between the browser and the server. By presenting counterfeit certificates to both the browser and the server, the malicious user could carry on two simultaneous encrypted sessions. Since the malicious user knows the secret on both connections, it is trivial to observe and manipulate data passing between the server and the browser. Server Man-in-the-middle User Figure 6.5: The man-in-the-middle effectively controls everything the user sees, and can record and manipulate all traffic. Without a public key infrastructure to verify the authenticity of keys, strong encryption alone cannot protect against this kind of attack. Use of a good PKI prevents this kind of attack. In order to be successful, the malicious user would have to present a certificate to the client that is signed by a trusted certificate authority. Unless a CA has been compromised (very unlikely) or the user is tricked into accepting the forged certificate, then such an attack is not possible. This is why it is vitally important that users understand that ignoring warnings about expired or improper certificates is very dangerous, especially when using wireless networks. By clicking the ignore button when prompted by their browser, users open themselves up to many potential attacks. SSL is not only used for web browsing. Insecure protocols such as IMAP, POP, and SMTP can be secured by wrapping them in an SSL tunnel. Most modern clients support IMAPS and POPS (secure IMAP and POP) as well as SSL/TLS protected SMTP. If your server does not provide SSL support, you can still secure it with SSL using a package like Stunnel ( SSL can be used to effectively secure just about any service that runs over TCP. SSH Most people think of SSH as a secure replacement for telnet, just as scp and sftp are the secure counterparts of rcp and ftp. But SSH is much more than encrypted remote shell. Like SSL, it uses strong public key cryptography to verify the remote server and encrypt data. Instead of a PKI, it uses a key fingerprint cache that is checked before a connection is permitted. It can use passwords, public keys, or other methods for user authentication. Many people do not know that SSH can also act as a general purpose encrypting tunnel, or even an encrypting web proxy. By first establishing an

15 Chapter 6: Security & Monitoring 171 SSH connection to a trusted location near (or even on) a remote server, insecure protocols can be protected from eavesdropping and attack. While this technique may be a bit advanced for many users, network architects can use SSH to encrypt traffic across untrusted links, such as wireless point-to-point links. Since the tools are freely available and run over standard TCP, any educated user can implement SSH connections for themselves, providing their own end-to-end encryption without administrator intervention. OpenSSH ( is probably the most popular implementation on Unix-like platforms. Free implementations such as Putty ( and WinSCP ( are available for Windows. OpenSSH will also run on Windows under the Cygwin package ( These examples will assume that you are using a recent version of OpenSSH. Internet All traffic sent from SSH server is unencrypted SSH Server SSH listens for a TCP connection on localhost port 3128 All web traffic is encrypted by SSH Web browser uses localhost port 3128 for its proxy Figure 6.6: The SSH tunnel protects web traffic up to the SSH server itself. To establish an encrypted tunnel from a port on the local machine to a port on the remote side, use the -L switch. For example, suppose you want to forward web proxy traffic over an encrypted link to the squid server at squid.example.net. Forward port 3128 (the default proxy port) using this command: ssh -fn -g -L3128:squid.example.net:3128 squid.example.net

16 172 Chapter 6: Security & Monitoring The -fn switches instruct ssh to fork into the background after connecting. The -g switch allows other users on your local segment to connect to the local machine and use it for encryption over the untrusted link. OpenSSH will use a public key for authentication if you have set one up, or it will prompt you for your password on the remote side. You can then configure your web browser to connect to localhost port 3128 as its web proxy service. All web traffic will then be encrypted before transmission to the remote side. SSH can also act as a dynamic SOCKS4 or SOCKS5 proxy. This allows you to create an encrypting web proxy, without the need to set up squid. Note that this is not a caching proxy; it simply encrypts all traffic. ssh -fn -D 8080 remote.example.net Configure your web browser to use SOCKS4 or SOCKS5 on local port 8080, and away you go. SSH can encrypt data on any TCP port, including ports used for . It can even compress the data along the way, which can decrease latency on low capacity links. ssh -fncg -L110:localhost:110 -L25:localhost:25 mailhost.example.net The -C switch turns on compression. You can add as many port forwarding rules as you like by specifying the -L switch multiple times. Note that in order to bind to a local port less than 1024, you must have root privileges on the local machine. These are just a few examples of the flexibility of SSH. By implementing public keys and using the ssh forwarding agent, you can automate the creation of encrypted tunnels throughout your wireless network, and protect your communications with strong encryption and authentication. OpenVPN OpenVPN is a free, open source VPN implementation built on SSL encryption. There are OpenVPN client implementations for a wide range of operating systems, including Linux, Windows 2000/XP and higher, OpenBSD, FreeBSD, NetBSD, Mac OS X, and Solaris. Being a VPN, it encapsulates all traffic (including DNS and all other protocols) in an encrypted tunnel, not just a single TCP port. Most people find it considerably easier to understand and configure than IPSEC. OpenVPN also has some disadvantages, such as fairly high latency. Some amount of latency is unavoidable since all encryption/decryption is done in user space, but using relatively new computers on either end of the tunnel can minimize this. While it can use traditional shared keys, OpenVPN

17 Chapter 6: Security & Monitoring 173 really shines when used with SSL certificates and a certificate authority. OpenVPN has many advantages that make it a good option for providing end-to-end security. Some of these reasons include: It is based on a proven, robust encryption protocol (SSL and RSA) It is relatively easy to configure It functions across many different platforms It is well documented It's free and open source. OpenVPN needs to connect to a single TCP or UDP port on the remote side. Once established, it can encapsulate all data down to the Networking layer, or even down to the Data-Link layer, if your solution requires it. You can use it to create robust VPN connections between individual machines, or simply use it to connect network routers over untrusted wireless networks. VPN technology is a complex field, and is a bit beyond the scope of this section to go into more detail. It is important to understand how VPNs fit into the structure of your network in order to provide the best possible protection without opening up your organization to unintentional problems. There are many good online resources that deal with installing OpenVPN on a server and client, we recommend this article from Linux Journal: as well as the official HOWTO: Tor & Anonymizers The Internet is basically an open network based on trust. When you connect to a web server across the Internet, your traffic passes through many different routers, owned by a great variety of institutions, corporations and individuals. In principle, any one of these routers has the ability to look closely at your data, seeing the source and destination addresses, and quite often also the actual content of the data. Even if your data is encrypted using a secure protocol, it is possible for your Internet provider to monitor the amount of data transferred, as well as the source and destination of that data. Often this is enough to piece together a fairly complete picture of your activities on-line. Privacy and anonymity are important, and closely linked to each other. There are many valid reasons to consider protecting your privacy by anonymizing your network traffic. Suppose you want to offer Internet connectivity to your local community by setting up a number of access points for people to connect to. Whether you charge them for their access or not, there is always the

18 174 Chapter 6: Security & Monitoring risk that people use the network for something that is not legal in your country or region. You could plead with the legal system that this particular illegal action was not performed by yourself, but could have been performed by anyone connecting to your network. The problem is neatly sidestepped if it were technically infeasible to determine where your traffic was actually headed. And what about on-line censorship? Publishing web pages anonymously may also be necessary to avoid government censorship. There are tools that allow you to anonymize your traffic in relatively easy ways. The combination of Tor ( and Privoxy ( is a powerful way to run a local proxy server that will pass your Internet traffic through a number of servers all across the net, making it very difficult to follow the trail of information. Tor can be run on a local PC, under Microsoft Windows, Mac OSX, Linux and a variety of BSD's, where it anonymizes traffic from the browser on that particular machine. Tor and Privoxy can also be installed on a gateway server, or even a small embedded access point (such as a Linksys WRT54G) where they provides anonymity to all network users automatically. Tor works by repeatedly bouncing your TCP connections across a number of servers spread throughout the Internet, and by wrapping routing information in a number of encrypted layers (hence the term onion routing), that get peeled off as the packet moves across the network. This means that, at any given point in the network, the source and destination addresses cannot be linked together. This makes traffic analysis extremely difficult. The need for the Privoxy privacy proxy in connection with Tor is due to the fact that name server queries (DNS queries) in most cases are not passed through the proxy server, and someone analyzing your traffic would easily be able to see that you were trying to reach a specific site (say google.com) by the fact that you sent a DNS query to translate google.com to the appropriate IP address. Privoxy connects to Tor as a SOCKS4a proxy, which uses hostnames (not IP addresses) to get your packets to the intended destination. In other words, using Privoxy with Tor is a simple and effective way to prevent traffic analysis from linking your IP address with the services you use online. Combined with secure, encrypted protocols (such as those we have seen in this chapter), Tor and Privoxy provide a high level of anonymity on the Internet. Network Monitoring Network monitoring is the use of logging and analysis tools to accurately determine traffic flows, utilization, and other performance indicators on a network. Good monitoring tools give you both hard numbers and graphical ag-

19 Chapter 6: Security & Monitoring 175 gregate representations of the state of the network. This helps you to visualize precisely what is happening, so you know where adjustments may be needed. These tools can help you answer critical questions, such as: What are the most popular services used on the network? Who are the heaviest network users? What other wireless channels are in use in my area? Are users installing wireless access points on my private wired network? At what time of the day is the network most utilized? What sites do your users frequent? Is the amount of inbound or outbound traffic close to our available network capacity? Are there indications of an unusual network situation that is consuming bandwidth or causing other problems? Is our Internet Service Provider (ISP) providing the level of service that we are paying for? This should be answered in terms of available bandwidth, packet loss, latency, and overall availability. And perhaps the most important question of all: Do the observed traffic patterns fit our expectations? Let's look at how a typical system administrator can make good use of network monitoring tools. An effective network monitoring example For the purposes of example, let's assume that we are in charge of a network that has been running for three months. It consists of 50 computers and three servers: , web, and proxy servers. While initially things are going well, users begin to complain of slow network speeds and an increase in spam s. As time goes on, computer performance slows to a crawl (even when not using the network), causing considerable frustration in your users. With frequent complaints and very low computer usage, the Board is questioning the need for so much network hardware. The Board also wants evidence that the bandwidth they are paying for is actually being used. As the network administrator, you are on the receiving end of these complaints. How can you diagnose the sudden drop in network and computer performance and also justify the network hardware and bandwidth costs?

20 176 Chapter 6: Security & Monitoring Monitoring the LAN (local traffic) To get an idea of exactly what is causing the slow down, you should begin by looking at traffic on the local LAN. There are several advantages to monitoring local traffic: Troubleshooting is greatly simplified. Viruses can be detected and eliminated. Malicious users can be detected and dealt with. Network hardware and resources can be justified with real statistics. Assume that all of the switches support the Simple Network Management Protocol (SNMP). SNMP is an application-layer protocol designed to facilitate the exchange of management information between network devices. By assigning an IP address to each switch, you are able to monitor all the interfaces on that switch, observing the entire network from a single point. This is much easier than enabling SNMP on all computers in a network. By using a free tool such as MRTG (see Page 190), you can monitor each port on the switch and present data graphically, as an aggregate average over time. The graphs are accessible from the web, so you are able to view the graphs from any machine at anytime. With MRTG monitoring in place, it becomes obvious that the internal LAN is swamped with far more traffic than the Internet connection can support, even when the lab is unoccupied. This is a pretty clear indication that some of the computers are infested with a network virus. After installing good anti-virus and anti-spyware software on all of the machines, the internal LAN traffic settles down to expected levels. The machines run much more quickly, spam s are reduced, and the users' morale quickly improves. Monitoring the WAN (external traffic) In addition to watching the traffic on the internal LAN, you need to demonstrate that the bandwidth the organization is paying for is actually what they are getting from their ISP. You can achieve this by monitoring external traffic. External traffic is generally classified as anything sent over a Wide Area Network (WAN). Anything received from (or sent to) a network other than your internal LAN also qualifies as external traffic. The advantages of monitoring external traffic include: Internet bandwidth costs are justified by showing actual usage, and whether that usage agrees with your ISP's bandwidth charges.

21 Chapter 6: Security & Monitoring 177 Future capacity needs are estimated by watching usage trends and predicting likely growth patterns. Intruders from the Internet are detected and filtered before they can cause problems. Monitoring this traffic is easily done with the use of MRTG on an SNMP enabled device, such as a router. If your router does not support SNMP, then you can add a switch between your router and your ISP connection, and monitor the port traffic just as you would with an internal LAN. Detecting Network Outages With monitoring tools in place, you now have an accurate measurement of how much bandwidth the organization is using. This measurement should agree with your ISP's bandwidth charges. It can also indicate the actual throughput of your connection if you are using close to your available capacity at peak times. A "flat top" graph is a fairly clear indication that you are operating at full capacity. Figure 6.7 shows flat tops in peak outbound traffic in the middle of every day except Sunday. It is clear that your current Internet connection is overutilized at peak times, causing network lag. After presenting this information to the Board, you can make a plan for further optimizing your existing connection (by upgrading your proxy server and using other techniques in this book) and estimate how soon you will need to upgrade your connection to keep up with the demand. This is also an excellent time to review your operational policy with the Board, and discuss ways to bring actual usage in line with that policy. Figure 6.7: A graph with a "flat top" is one indication of overutilization.

22 178 Chapter 6: Security & Monitoring Later in the week, you receive an emergency phone call in the evening. Apparently, no one in the lab can browse the web or send . You rush to the lab and hastily reboot the proxy server, with no results. Browsing and are still broken. You then reboot the router, but there is still no success. You continue eliminating the possible fault areas one by one until you realize that the network switch is off - a loose power cable is to blame. After applying power, the network comes to life again. How can you troubleshoot such an outage without such time consuming trial and error? Is it possible to be notified of outages as they occur, rather than waiting for a user to complain? One way to do this is to use a program such as Nagios that continually polls network devices and notifies you of outages. Nagios will report on the availability of various machines and services, and will alert you to machines that have gone down. In addition to displaying the network status graphically on a web page, it will send notifications via SMS or , alerting you immediately when problems arise. With good monitoring tools in place, you will be able to justify the cost of equipment and bandwidth by effectively demonstrating how it is being used by the organization. You are notified automatically when problems arise, and you have historical statistics of how the network devices are performing. You can check the current performance against this history to find unusual behavior, and head off problems before they become critical. When problems do come up, it is simple to determine the source and nature of the problem. Your job is easier, the Board is satisfied, and your users are much happier. Monitoring your network Managing a network without monitoring is similar to driving a vehicle without a speedometer or a fuel gauge, with your eyes closed. How do you know how fast you are going? Is the car consuming fuel as efficiently as promised by the dealers? If you do an engine overhaul several months later, is the car any faster or more efficient than it was before? Similarly, how can you pay for an electricity or water bill without seeing your monthly usage from a meter? You must have an account of your network bandwidth utilization in order to justify the cost of services and hardware purchases, and to account for usage trends. There are several benefits to implementing a good monitoring system for your network: 1. Network budget and resources are justified. Good monitoring tools can demonstrate without a doubt that the network infrastructure (bandwidth, hardware, and software) is suitable and able to handle the requirements of network users.